If you're going for completion of the DLC, you should not sink any time into grinding this achievement until you have completed all of the following achievements:
PLUS, you should have enough victories to have unlocked the CLEO heavy sniper rifle in the State of Decay 2 story mode, so that you can unlock:
Once you have done all of the above, you'll either have already earned this achievement or you are ready to grind it out the rest of the way. Do so by repeatedly starting a new game, completing either one or two waves, quitting to the main menu, and then starting a new game. You can complete a wave every four minutes this way. This is how it breaks down:
Wave 1 - almost 2 minutes of menus, setup, loading screens, talking, etc. followed by a 2-minute wave of zombies.
Wave 2 - 1 minute of intermission followed by a 3-minute wave of zombies.
It is up to you whether you grind out 1-wave or 2-wave games. The 1-wave games might go slightly
quicker than 4 minutes, like maybe 3:50 or 3:45 minimum, so for ultimate efficiency, there is a little bit of an edge to doing it that way. (Note that you will actually have to fight. Unfortunately, if you repair the wall at the beginning of Wave 1 and then go AFC, the zombies are going to break through. Between them and the feral, your technician may not survive.) Then again, some players may find finishing Wave 2 before restarting to be a little more enjoyable.
It takes another 6 minutes to complete Wave 3, which is inefficient.
Each percentage point on the progress indicator equals 3.43 waves, so at 4 minutes per wave, you're looking at about 13.7 minutes per percentage point. That equals 2 hours, 17 minutes for every 10 percentage points.